This invention relates generally to wireless telecommunications systems and, more particularly, to digital radio communications system that employ space-time encoding of bits to symbols.
In conventional digital radio communications systems data bits to be transmitted to a receiver are processed by first mapping a block of K bits to a data symbol (bit-to-symbol mapping), where for a set of K bits 2K symbols are required. The symbol represents one specific point in a signal constellation, also referred to as a signal space diagram. In most cases the data bits have been encoded by a channel encoder and, in some cases, the symbols obtained after the bit-to-symbol mapping are then operated on by a further encoder. One example is so-called space-time coding.
In a typical implementation well-known Gray encoding is used for the bits-to-symbol mapping (see, for example, Proakis: Digital Communications, 3rd Edition, pages 175-178, and FIG. 2 herein.) After Gray encoding, adjacent constellation points in the signal space diagram are assigned bit combinations that differ in only one bit position. This is advantageous, as in traditional transmission methods the most probable error is made by selecting a symbol that is closest to the correct symbol (in Euclidean distance), and thus only a one bit error results from the selecting the wrong symbol.
In the space-time coding approach the data symbols are encoded such that an input data symbol stream generates one encoded symbol stream for transmission from individual ones of a plurality of antennas. At a receiver antenna, the received signal contains the sum of the signals transmitted from the plurality of antennas, each being affected by the radio channel. In addition, the received signal is typically corrupted by noise and interference. The receiver operates to generate symbol decisions on the data symbols using the signal received by the receiver antenna(s).
In general, the signal constellation is defined by the modulation that is employed (e.g., 8-PSK), and the space-time codes that are employed are designed for the selected type of modulation and, thus, for the corresponding signal constellation.
The Gray encoding used for bit-to-symbol mapping is, however, not necessarily always the most optimum method for use in a space-time coding embodiment. However, the inventors are not aware of any superior technique that has been proposed to replace Gray encoding.
General reference with regard to space-time coding can be made to WO 9741670 A1, which also discusses code construction. However, this publication does not specifically address the problem of bit-to-symbol mapping and, in fact, an unoptimized bit-to-symbol mapping appears to be used.
It is a first object and advantage of this invention to provide an improved technique for implementing bit-to-symbol mapping for use in a space-time encoder.
It is a further object and advantage of this invention to provide a bit-to-symbol mapping technique that is optimized for a particular application, and that may consider, for example, actual symbol selection errors, a type of modulation that is in use, and radio channel characteristics.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed for optimizing a bit-to-symbol mapping operation. The method has steps of (a) determining a most probable symbol selection error made during a space-time decoding operation; and (b) selecting a bits-to-symbol mapper such that a bit-to-symbol mapping step results in a most probable symbol selection error, made during a space-time decoding operation, causes a minimal number of bit errors. The bits-to-symbol mapping step can be carried out so as to minimize an average number of bit errors resulting from an occurrence of the most probable symbol selection error, as well as from an occurrence of at least a second most probable symbol selection error, during the space-time decoding operation. Even further optimizations based on third most probable symbol selection errors, fourth most probable symbol selection errors, etc., may also be done.
The bits-to-symbol mapping operation is preferably followed by a space-time encoding operation.
The step of determining the most probable symbol selection error can include at least one of a step of modelling a communications channel, as well as the receiver, and/or considering a type of modulation used when transmitting the bits-to-symbol mapped signal through a communications channel. A space-time coded communications system that operates in accordance with the method is also disclosed.